Fluke Biomedical 956A-201-M2 - Manual (Page 85)

5-7
1. Disconnect the interconnection cable at the detector.
2. Remove the six (6) screws around the circumference of the housing.
3. Extract the preamplifier assembly, taking care no to damage the detector-housing gasket.
Detector Assembly (Factory only)
1. Place the preamplifier assembly in the protective housing and re-align the detector-housing gasket or
o-ring. Replace gasket or o-ring if damaged or brittle.
2. Replace the screws around the circumference of the housing.
3. Connect the interconnection cable.
Discriminator Setpoint Adjustment (Factory Adjustment Only)
This adjustment provides discrimination of input pulses that are below a user specified value. The nominal
setpoint value (factory set) is +1.0 Vdc. If a value other than +1.0 Vdc is desired, adjustment can be made
by connecting a DVM to the node of R38 - R39 (+) and R44 (-). Turn potentiometer R44 to the level of
pulse discrimination. The adjustment is then complete.
5.2 HV POWER SUPPLY ADJUSTMENT
R5 adjusts the detector high voltage, and the HV test jack allows a 1000:1 voltage measurement to be
read. For the Model 956A-201 Universal Digital Ratemeter, refer to the appropriate detector calibration
data sheet included with each detector for specific high voltage settings.
Anti-jam Setpoint Adjustment (Factory Adjustment Only)
When the detector is used in a digital system, the parameters (dead time correction, count conversion, and
anti-jam setpoint) are determined during factory calibration. Therefore, no customer adjustments are
required. If the G-M tube fails, the detector must be returned to the factory for repair and re-calibration.
DC Power Supply, Detector (Factory Test Only)
AC voltage is present on the printed circuit board (pads J & H).
Exercise caution when making measurements near this area.
1. With cable connected to the UDR, turn on the UDR to provide a low voltage power supply to the
detector. High voltage is to be disconnected.
2. Using a DVM, measure the voltage at pad A (+ DC input Voltage) and pad c; chassis ground (-). The
reading should be +10 Vdc (analog readouts) or +15 Vdc (digital readouts). If proper voltage is
obtained, measure the voltage present across C1. The reading should be 10 Vdc
± 0.2 V for the 10-
volt power supply. If channel power is 15 Vdc and the reading obtained in this step is greater than
+10.5 V, check jumper J3 for proper placement (i.e. A/B, 15 V; B/C, 10 V). If voltage is not present at
pad A, remove the connector at the detector and inspect the cable assembly for broken or cut wires.
3. Measure the +15 Vdc power supply at the UDR to ensure that the UDR is functional. If not, refer to
the applicable drawings for the UDR so that the appropriate troubleshooting procedure can be used.
4. If +10 V is not present across capacitor C1, preamplifier circuitry failure has occurred. Using the
schematic diagram as a guide, analyze and replace the faulty components.
WARNING